Approximately 14.5 billion pounds of propylene oxide (PO) are produced per year. Propylene oxide has many uses. Between 60 and 70% of the propylene oxide is converted to polyether polyols for the production of polyurethane plastics. About 20% of propylene oxide is hydrolyzed into propylene glycol, via a process which is accelerated either by thermal reaction or by acid or base catalysis. Other major products are polypropylene glycol, propylene glycols ethers, and propylene carbonate. To produce these end products, propylene oxide substantially free of impurities is needed.
Methods of producing alkylene oxides including propylene oxide involve hydrochlorination, direct oxidation and epoxidation of its corresponding olefins by peroxide or hydroperoxide. The oxidates used in the epoxidation processes are derived from secondary or tertiary hydrocarbons by direct oxidation with molecular oxygen; hence, the oxidates contain oxygenate impurities and precursors. Additional oxygenate impurities are also generated in the step of epoxidation of olefins. Crude alkylene oxides, such as propylene oxide, particularly those produced via epoxidation with hydrocarbon oxidates, contain amounts of oxygenated impurities that are difficult to separate from alkylene oxides. The impurities may include water, acids, alcohols, aldehydes, alkanes, ketones and esters. A need exists for continued improvement of systems and methods for separating alkylene oxide from these impurity constituents of crude alkylene oxide streams.
Although the purity of crude propylene oxide, for example from a propylene oxide and tertiary butyl alcohol (PO/TBA) process, can be as high as 98.5%, the crude propylene oxide generally contains close-boiling impurities including, without limitation, one or more of water, methanol, methyl formate, formaldehyde, acetaldehyde, acetone, propionaldehyde, isobutylene oxide, aldehyde derivatives and C5-C7 hydrocarbons. To meet commercial grade product propylene oxide specification, the impurities are removed from the crude propylene oxide. Due to close-boiling points, these impurities are difficult to separate from the propylene oxide without using an elaborate propylene oxide refining or purification scheme that involves extractive distillation techniques.
Traditional propylene oxide purification includes the production of purge streams comprising propylene oxide (also referred to as ‘slop propylene oxide cuts’) that may equal from 18 to 22 weight percent of the propylene oxide entering the refining section in the crude propylene oxide. Such purge streams comprising propylene oxide are conventionally utilized to make propylene glycol (PG) to capture added value with reduced equipment and energy costs in the propylene oxide refining/purification section. Reduction of PO losses in purge stream can provide for greater overall recovery of product PO, which may be desirable relative to making PG.
It remains a challenge to recover a purified propylene oxide product containing low levels of impurities, such as aldehydes and alcohols, particularly for propylene oxide produced from a free-radical oxidation process, including for example tert-butyl hydroperoxide processes. Accordingly, improved systems and methods are needed for recovering propylene oxide, from effluent streams of various crude propylene oxide production methods, in a high state of purity without excessive loss of propylene oxide product.